1. Field of the Invention
The present invention relates to an apparatus for reproducing information from an optical disk and, more particularly, to an improved optical unit for use in focusing laser light to form an image on the information recording surface of an optical disk.
2. Background of the Invention
A conventional apparatus for use in recording and reproducing information from an optical disk is described in Unexamined Published Japanese Patent Application Ser. No. 143443/1983. This apparatus uses an optical unit for focusing the laser light emitted from a semiconductor laser to form an image on the information recording surface of the disk. This optical unit is provided with a parallelepiped plate that allows the angle formed between a line normal to the laser light transmitting surface of the plate and the optical axis of the laser light to be varied within a plane containing the junction plane of the semiconductor laser.
The purpose of providing a parallelepiped plate is as follows. A semiconductor laser typically emits light along its p-n junction plane and the state of convergence of laser light within a plane containing the junction plane differs from that of convergence within a plane vertical to that plane. If no correction is taken, the laser light will cause astigmatism at the position where proper focusing should occur and the resulting distortion in the image of the beam spot precludes faithful reproduction of information. A parallelpiped plate is employed in order to avoid this problem.
In this instance, in order to achieve accurate scanning of laser light on the information recording surface of the disk, a diffraction grating must be disposed in the optical unit so that a tracking error signal is attained from the diffracted light.
As shown above, the image-forming optical unit in a conventional apparatus for recording and reproducing information from an optical disk requires a diffraction grating to be provided as a component separate from the parallelepiped plate. However, this places inevitable limitations on the effort to decrease the space occupied by the apparatus, and the number of components employed in great enough to increase the manufacturing cost of the apparatus.